The present invention relates to a process for producing a buried strip semiconductor laser and to a laser obtained by this process. It has a general application in optical telecommunications.
The technical field of the invention is that of Ga.sub.1-x In.sub.x As.sub.1-y P.sub.y or InP semiconductor laser sources for single-mode fibre links. The invention more particularly relates to a structure having a so-called buried strip geometry, which is a structure with a low threshold current. This structure has interesting advantages compared with the prior art structures. It firstly makes it possible to minimize the leakage currents appearing at high operating temperatures and/or high optical powers. It also permits a simple production process in two instead of three epitaxy stages, such as are required in most known structures.
The attached FIG. 1 shows a known structure with distributed feedback and buried strip. This is a Japanese structure of the NEC Corporation described in the article entitled: "Low threshold and high temperature single longitudinal mode operation of 1.55 .mu.m--Band DFB-DC-PBH-LD's" by M. Kitamura et al published in Electronics Letters, July 5 1984, pp 596-597. This structure comprises a n-doped InP substrate 10, a InGaAsP active layer 12, a InGaAsP guide layer 14, a p-doped InP confinement layer 16, a p-doped -InP layer 18, a n-doped Inp layer 20, a p-doped InP layer 22 and finally a p-doped InGaAsP contact layer 24.
The distributed feedback structure or DFB comprises a holographic network 28 of the first or second order (spacing 0.24 or 0.48 .mu.m) etched in the guide formed by the layer 14 located just above the active layer 12, which functions at a wavelength of 1.55 .mu.m. The electronic confinement of the current lines in the strip is obtained with the aid of crescent-shaped p and n-like layers 26, located on either side of the active zone 12 of the laser.
The best results at a wavelength of 1.3 and 1.55 .mu.m were obtained with this structure: threshold current 20 mA and power of 10 mW per face up to 60.degree.C.
This structure is produced in three epitaxy stages: the first for depositing the two quarternary layers 12, 14, the second for repeating the epitaxy of a InP layer 16 on the network previously etched in layer 14, and the third for the epitaxy of the n and p type InP blocking layers 20, 22 and that of the InGaAsP contact layer 24.
Reference is made to the high degree of precision required for the positioning of the blocking layers with respect to the active zone. It is also pointed out that this structure can only be produced by liquid phase epitaxy.
W. T. TSANG et al of Bell Laboratories recently published an article entitled "Heteroepitaxial ridge-overgrown distributed feedback laser at 1.5 .mu.m" in Appl. Phys. Lett., 45, Dec. 15, 1984, pp 1272-1275, where a description is given of a so-called DFB-HRO structure (Distributed Feedback-Heteroepitaxial Ridge Overgrown), which can be an interesting replacement solution. The corresponding structure is illustrated in FIG. 2. It comprises a n-doped InP substrate 30, a n-doped InP buffer layer 32, a n-doped GaInAsP active layer 34, a p-doped GaInAsP anti-redissolving layer 36 in which is provided a defractive network 38, a SiO.sub.2 dielectric film 40 having an opening in which has been formed a p-doped InP band (Ridge Overgrowth) 42, said band being covered by a metal coating.
In such a structure, the active strip is not really buried. The electric confinement of the current is obtained by the opening (5 .mu.m wide) made in the silica layer. The optical light guide produced in the active layer is much more effective than in the buried structure of FIG. 1. Therefore the threshold currents are much higher.
It should be noted that this structure is produced in two epitaxy stages:
the first for depositing the three InP layers and the quaternary layers,
the second for the p-doped InP confinement layer obtained by localized epitaxy through the SiO.sub.2 mask 40 previously deposited on layer 36.
Another structure is known from the publication of the present Applicants and is entitled: "1.55 .mu.m strip buried Schottky laser" published in "Proceedings of the 9th IEEE International Semiconductor Laser Conference", Aug. 7/10 1984. This structure is shown in FIG. 3 and comprises a n.sup.+ -doped InP substrate 50, a n-doped InP confinement layer 52, an approximately 2 .mu.m wide strip formed from a GaInAsP active layer 54 and a Ga InAsP guide layer 55, said strip being buried in a p-doped InP layer 56. This layer is covered with a GaInAs layer 58 in the form of an approximately 5 .mu.m wide mesa. The assembly is covered with a layer of titanium 60 and gold 62.
This so-called SBH structure (strip buried heterostructure) is formed in two epitaxy stages. The first makes it possible to deposit the three layers 52, 54, 55, respectively of InP and GaInAsP of composition corresponding to a wavelength of 1.55 .mu.m and GaInAsP at 1.3 .mu.m, said layers then being etched by selective etching in strip form. The second epitaxy cycle makes it possible to grow again on the strip the type p InP layer 56 and a contact layer 58. The confinement of the current on the mesa is then obtained by depositing on the apex a Schottky-type strip.
The SBH structure makes it possible to produce in this way a buried strip DFB laser without any supplementary complication. It is merely necessary to additionally etch a holographic network in the layer 55 functioning at 1.3 .mu.m and then repeat epitaxy taking care not to redissolve either the network or the strip.
A DFB-SBH laser produced according to this method is described in the article entitled "Laser Ga.sub.x In.sub.1-x As.sub.1-y P.sub.y 1.55 .mu.m a contre-reaction distribuee" by P. CORREC et al in Comptes-Rendus des Journees Nationales de l'Optique Guidee, Mar. 20/21 1985, Issy-les-Moulineaux, France.
Although being satisfactory in certain respects, all these structures suffer from disadvantages.
The DC PBH type strip structure (FIG. 1) apply to DFB lasers is very difficult to realize, particularly due to the three successive epitaxies and the accuracy required for the etching thicknesses and depths.
The HRO-DFB-type structure (FIG. 2) is simpler than the first, but leads to higher threshold currents (50-100 mA instead of 20-40 mA) due to the limited electronic and optical lateral confinement.
The SBH structure gives rise to production difficulties, if it is wished to obtain a distributed Bragg network, due to the liquid phase epitaxy leading to a partial redissolving of the guide layer during epitaxy repeat. There are also high leakage currents in the p-n junctions of the InP on either side of the strip, starting from a total current of 80 mA. Thus, there is a limitation to the emission power of the laser (3 mW at 60.degree. C. instead of 10 mW with the DC-PBH structure). Limitations of the same order of magnitude are also observed in lasers produced by MO-CVD epitaxy ("Metal-Organic, Chemical Vapor Deposition").